P3D Re: Apochromatic lenses More info please

["John Goodman" <jgood@xxxxxxxx>]

> >"apochromatic
> >lenses are made of crystalline fluorite or other unusual types
> >of glasses, have stronger elements (higher curvature)"
> >    
> >Could you explain what this type of glass does better than other
> >types of glass and what you mean by higher curvature?
> 
> Sorry, I don't know more than the passage quoted from Kinglake's
> "Optics in Photography".  The "higher curvature" was mine which
> I think is a result of "stronger elements".  Stronger elements =
> shorter focal length = higher curvature.

I only have a layman's understanding of this, but a missing 
term important for the above may be "index of refraction". 
This is a measure of the speed of light in the material as 
compared with the speed of light in a vacuum. Materials 
with a higher refractive index "bend" light more strongly. Of 
course the curvature, or shape, of a lens element is a 
major factor in how strongly light is bent, but sophisticated 
lens designs utilize both the physical curvature of the 
elements and the refractive properties of a combination of 
different glasses to achieve superior results. Fluorite 
glasses I believe offer the lens designer certain 
advantages, but at a price (monetary cost, fabrication 
difficulties, environmental considerations).

JG


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